Traditional mechanical speedometers are connected to the transmission output shaft by a gear driven flexible rotating shaft enclosed in a cable. Except for a change in axle ratio, the speedometer cable rotation is then directly proportional to the road speed. The proportion is determined by the ratio between a transmission drive gear and the mating cable gear, as well as the tire diameter and rear axle differential ratio. The speedometer unit is attached to the other end of the speedometer cable and provides a visual indication of speed from the speedometer cable rotation according to a second fixed proportion. Since a manufacturer may make vehicles which have a rear axle ratios that vary over a nearly 2-to-1 range, and select tires which have a difference in wheel revolution per mile of nearly 20 percent (between the largest to the smallest tire), the manufacturer must stock a wide variety of speedometers or speedometer cable gears to provide the proper speed and odometer readings. Moreover, problems of mechanical failure and routing of the speedometer cable within the body of the vehicle require significant time and effort to resolve with each different vehicle produced.
In electric speedometers, a magnetic pick-up is placed close to the speedometer cable drive gear in the transmission housing to produce a periodic signal that is proportional to the vehicle speed. The periodic signal is received by an electric or electronic speedometer unit having a fixed relationship between the periodic signal and the indicated vehicle speed and distance travelled. In such speedometer units, the vehicle manufacturer must stock a unique speedometer unit for each tire size and/or axle ratio. Adjustments are made within the speedometer unit itself, and require precision adjustment and calibration of the ratio between the indicated velocity and the period of the received signal. Moreover, a separate adjustment of the odometer range is necessary. The adjustments require sophisticated operator training, tools, and calibration equipment not normally available outside of the factory.
In special purpose or in heavy duty commercial vehicles, the rear axle (differential) often includes a second gear ratio which is selectable by the vehicle operator. In order for the mechanical speedometer to accommodate the additional axle ratio, additional mechanical gear ratios are required which correspond to the appropriate rear axle ratio. Electric speedometers typically include a divider to operate the odometer counter. To accommodate a second rear axle ratio would require a counter having a second divider modulus, which second modulus is likely to be inconvenient for available digital counters.